1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same and, more particularly, to a semiconductor device having a non-monocrystalline semiconductor layer represented by an amorphous silicon, and a method of manufacturing the same.
2. Related Background Art
In a conventional image information processing apparatus such as a facsimile or an image reader, a photosensor is used as a photoelectric converter. In particular, in recent years, a high-sensitive image reading apparatus having a long line sensor obtained by one-dimensionally arranging photosensors using a hydrogenated amorphous silicon as a photoelectric conversion layer is proposed. Further, a reading apparatus in which photosensors are two-dimensionally arranged in a large area, and a high-performance image reading apparatus in which thin-film transistors or shift resisters are formed on the same substrate by using a hydrogenated amorphous silicon are provided. In particular, in recent years, liquid crystal display apparatuses which are driven by thin-film transistors using a hydrogenated amorphous silicon to cope with large screens are actively developed and manufactured.
FIG. 1 shows an example of an element arrangement of a one-dimensional image reading apparatus. Referring to FIG. 1, reference symbol SR1 is a shift resister; S1, a photosensor portion; C1, a capacitor portion for accumulating charges; TFT1, a transfer TFT for transferring accumulated charges; and Sig MTX1, a signal line matrix wiring for outputting the transferred charges out of the circuit. A one-dimensional image reading apparatus is constituted such that such elements are arranged at 1,728 bits in A-B direction for, e.g., A4 size. Note that reference symbols .phi.1 and .phi.2 denote block lines, and reference symbol V.sub.DD denotes a power supply.
FIG. 2 shows an example of a sensor portion of an image reading apparatus as a C-D section of the element arrangement shown in FIG. 1. FIG. 2 shows only the photosensor portion S1. A gate electrode 2 is formed on a transparent insulating substrate 1, and an insulating layer 3 consisting of SiO.sub.2, SiN.sub.x, or the like, a semiconductor layer 4 consisting of an amorphous silicon, a doping semiconductor layer 5 such as an n.sup.+ -type amorphous silicon, and a main electrode 6 are formed on the gate electrode 2 by patterning as needed. Reference numeral 18 denotes an anti-abrasion thin glass, having a thickness of, e.g., 50 .mu.m, for preventing abrasion of an image reading element by an original surface; and 17, a surface of the element. The surface 17 is constituted by a layer which serves as a protective layer as needed and consists of SiN, and a polyimide resin, and an epoxy resin for adhering the anti-abrasion thin glass 18. In this arrangement, light rays 20 emitting from a light source 22 such as an LED pass through an illumination transmission window 21 of the transparent insulating substrate 1 and are reflected by the original surface 19. The reflected light rays are incident on the TFT photosensor portion S1. The photosensor portion S1 generates a light output depending on the intensity of the reflected light rays as an electric signal, so that an image can be processed with gradation.
FIGS. 3 to 5 show examples of the arrangement of a thin-film transistor. Referring to FIG. 3, a transparent or opaque conductive layer is patterned on the transparent insulating substrate 1 to form the gate electrode 2, and the insulating layer 3 consisting of SiO.sub.2 or SiN.sub.x, the semiconductor layer 4 consisting of an amorphous silicon, and the doping semiconductor layer 5 are formed on the gate electrode 2 by patterning as needed. A discrete electrode 7 and the main electrode 6 are formed by patterning.
FIG. 4 shows an example wherein a channel protective layer 8 is formed on the semiconductor layer 4 in FIG. 3.
FIG. 5 shows the following example. That is, a light-shielding layer 9 and an insulating layer 10 are formed on the transparent insulating substrate 1 by patterning as needed, and the discrete electrode 7 is formed on the resultant structure by patterning. The main electrode 6 and the doping semiconductor layer 5 are formed with a gap by patterning, and the semiconductor layer 4 and the insulating layer 3 are sequentially formed on the gap. The gate electrode 2 is formed on the resultant structure by patterning.
A large-area two-dimensional sensor driven by a thin film transistor using a silicon has developed.
FIG. 6 is a typical sectional view showing a large-area two-dimensional sensor, corresponding to one pixel, for an apparatus for detecting an electromagnetic wave including radiation such as X-rays or light rays. Referring to FIG. 6, a thin-film transistor (T11) having an electrode 62 serving as a gate electrode, an insulating layer 63 serving as a gate insulating layer, a hydrogenated amorphous silicon semiconductor layer 64, a doping semiconductor layer 65, and an electrode serving as a main electrode, an MIS photosensor (S11) having the electrode 62 serving as a lower electrode, the insulating layer 63, the hydrogenated amorphous silicon semiconductor layer 64, and the doping semiconductor layer 65, and a capacitor (C11) having the electrode 62 serving as a lower electrode, the insulating layer 63, the hydrogenated amorphous silicon semiconductor layer 64, the doping semiconductor layer 65, and the electrode 66 serving as an upper electrode are parallelly arranged and formed on an insulating substrate 61 to constitute one pixel.
Such pixels are arranged in a two-dimensional large area, and a protective layer 68, consisting of SiN., for protecting the respective pixels and a phosphor 69 for converting the incident radiation such as X-rays into visible light rays are formed on the pixels, thereby constituting a so-called radiation detecting apparatus which copes with a large area.
However, the apparatus with the above arrangement has the following points to be improved.
In the arrangement shown in FIG. 3, the doping semiconductor layer 5 formed in a gap K in FIG. 3 must be removed by etching to a channel of a thin-film transistor after the doping semiconductor layer 5 or/and the main electrode 6 are formed. At this time, a dopant contained in the doping semiconductor layer 5, e.g., phosphorus atoms, are partially diffused in the semiconductor layer 4 in formation of the doping semiconductor layer 5. For this reason, the diffused layer must be also removed at the same time. In this case, in particular, if the transparent insulating substrate 1 has a large area, the end point of slight etching of the semiconductor layer 4 cannot be determined. For this reason, a disadvantage that the semiconductor layer 4 is over-etched or etched with poor uniformity may be posed. As a result, the characteristics of the thin-film transistor, especially, a voltage V.sub.th and a variation thereof, the value of electron mobility and a variation thereof, and a V.sub.th shift in an operation reliability test and a variation thereof may become worse. In addition, although no t shown, upon completion of the step of finally protecting the structure with a passivation film, the surface of the semiconductor layer 4 is temporarily exposed to the atmospheric air. Due to this influence, the above problems may become worse, or a change in temperature characteristics may disadvantageously vary.
In contrast to this, in the arrangement shown in FIG. 4, the above problems can be regulated. The number of masks used in the steps in manufacturing the arrangement in FIG. 4 is larger than that in FIG. 3. The thin-film transistor in FIG. 4 cannot be easily manufactured at a cost lower than that of the thin-film transistor in FIG. 3.
In the arrangement shown in FIG. 5, since the semiconductor layer 4 is formed after the main electrode 6 and the doping semiconductor layer 5 are formed by patterning, the ohmic contact of the semiconductor layer 4 cannot be easily obtained, and preferable TFT characteristics may be not easily obtained. In addition, since the insulating layer 3 is formed after the semiconductor layer 4 is formed, when the formation temperature is set to 350.degree. C. to form the insulating layer 3 having high quality, the semiconductor layer 4 which has been formed is degraded. When the formation temperature of the insulating layer 3 decreases, the insulating layer 3 having high quality cannot be obtained, and transistor characteristics, especially, ON-OFF characteristics, a voltage V.sub.th, and a V.sub.th shift, may be degraded.
Further, as shown in FIG. 6, the doping semiconductor layer 5 for ohmic contact is formed on the thin-film transistor (T11) and the MIS photosensor (S11) using a hydrogenated amorphous silicon semiconductor. In this case, a micropatterning process by photolithography using a photomask must be performed.
To decrease the number of steps, the doping semiconductor layers 5 of elements, i.e., the thin-film transistor (T11) and the MIS photosensor (S11), have the same film thickness. However, in the thin-film transistor (T11), the doping semiconductor layer 5 desirably has a low resistivity to improve its transfer capability. On the other hand, in the MIS photosensor (S11), since the doping semiconductor layer 5 serves as a transparent electrode, the doping semiconductor layer desirably has a low resistivity and a small thickness to cause light rays to sufficiently reach the hydrogenated amorphous silicon semiconductor layer 4.
To make this point clear, FIG. 7 shows a film thickness dependency of the resistivity of the doping semiconductor layer formed by a normal P-CVD method. As is apparent from FIG. 7, when the doping semiconductor layer is formed to have a small thickness, its resistivity increases. In particular, when the thickness is decreased from 500 .ANG. to 300 .ANG., the resistivity increases to several hundreds .OMEGA..multidot.cm.
FIGS. 8 and 9 show, with respect to the thin-film transistor and the MIS photosensor, dependencies of a transfer capability and a light output on the formation film thickness of the doping semiconductor layer 5 formed by the normal P-CVD method, respectively. As is apparent from FIGS. 8 and 9, in the thin-film transistor, when the doping semiconductor layer 5 decreases in thickness, the increase in resistance of the doping semiconductor layer 5 degrades the transfer capability. In the photosensor, the light output increases with a decrease in thickness of the doping semiconductor layer 5. However, a doping semiconductor layer having a thickness: 300 .ANG. is used, the resistance of the transparent electrode is too high so that the light output cannot be measured.
For these reasons, in the prior art, at any rate, the thickness of the doping semiconductor layer 5 is set to 750 .ANG., and the respective functions are realized within the satisfactory range. However, decreases in resistance and thickness of the doping semiconductor layer 5 are desired to realize higher functional transistor and photosensor.
In addition, the doping semiconductor layer 5 is deposited to have a large thickness, and only the doping semiconductor layer 5 on the MIS photosensor is etched by the photolithographic step. In this manner, the thickness of the doping semiconductor layer is set to 1,000 .ANG., and the thickness of the doping semiconductor layer 5 of the MIS photosensor is set to 500 .ANG., thereby obtaining a high transfer capability and a high light output at once. In this case, since the photolithographic step is additionally performed, a problem that a yield of semiconductor devices increases and a problem that the production cost of the semiconductor devices increases are posed. Further, a problem that, on a substrate having a large area, e.g., a 460-mm square, the uniformity of etching cannot be sufficiently, technically assured is also posed, thereby preventing the photosensor and TFT from being realized.
The photosensor and TFT are formed in the same processes. The thickness of the semiconductor layer 4 of the photosensor must be large to assure the high light output of the photosensor. For this reason, the semiconductor layer 4 used on the TFT side increases in thickness in the same manner as in the photosensor, and the TFT may be erroneously operated by irradiation of light onto the TFT. Therefore, a light-shielding film is required for the TFT, so that a problem of an increase in number of steps and a problem of an increase in cost are disadvantageously posed.